Chemical Composition and Ferrite FN Analysis of 90mm Wide Belt Extreme Cladding Austenitic Stainless Steel

Overview and Background

Wide belt cladding (also referred to as strip cladding or hot-strip rolling cladding) of austenitic stainless steel onto carbon steel or low-alloy steel substrates is a critical process in the fabrication of corrosion-resistant pressure vessels, heat exchangers, and storage tanks. The topic under review focuses on the chemical composition control and ferrite number (FN) analysis of austenitic stainless steel deposited by a 90 mm wide belt extreme cladding process. This width represents a significant departure from conventional narrow-strip or single-pass weld overlay methods, demanding precise thermal management, dilution control, and microstructure homogeneity across the entire clad width.

The 90 mm belt width implies that the process may involve multi-pass deposition or a high-deposition-rate single-pass technique using submerged arc welding (SAW) with flux-cored wire, or possibly a specialized strip-feed system. The key challenge lies in maintaining uniform ferrite content across the full width while preventing excessive dilution from the base metal, which can shift the microstructure away from the desired fully austenitic or austenite-ferrite dual-phase condition.

Key Technical Points on Chemical Composition Control

The chemical composition of the overlay material is the primary determinant of corrosion resistance, mechanical properties, and weldability. For austenitic stainless steel cladding layers intended for hydrogenation service, acid environments, or chloride-containing media, the following composition ranges are typically targeted:

A critical observation from the literature is that dilution from the carbon steel base metal introduces carbon and manganese into the overlay, which can promote martensite formation in the heat-affected zone (HAZ) and reduce the ferrite number below the acceptable threshold. For 90 mm wide belts, the dilution profile is not uniform across the width; the edges tend to experience higher dilution due to lateral heat flow and reduced thermal mass at the boundaries.

Ferrite Number Analysis and Microstructure

Ferrite number (FN), as defined by ISO 8044 and ASTM E1243, is a quantitative measure of the delta ferrite content in austenitic stainless steel weld metal. The target FN range for most corrosion-resistant applications is 5–35 FN, with a preferred range of 10–20 FN for optimal balance of hot cracking resistance and corrosion performance.

The Schaeffler diagram (iron equivalent vs. chromium equivalent) provides a first-order prediction of weld metal microstructure. For a typical 316L-type cladding composition, the Cr equivalent (Cr + Mo + 1.5Si + 3N) falls in the range of 24–28, while the Ni equivalent (Ni + 30C + 0.5Mn) falls around 10–14, placing the composition in the austenite-plus-ferrite region. However, dilution from the base metal shifts the effective composition toward the ferrite-rich side.

The 90 mm width presents a unique challenge: the thermal cycle varies significantly from the center to the edges of the deposited belt. At the center, the peak temperature is higher and the cooling rate is slower due to greater thermal accumulation, which can promote grain growth and carbide precipitation. At the edges, the cooling rate is faster due to lateral heat dissipation into the base metal, which can lead to higher ferrite content and potentially martensite formation.

The literature emphasizes that the FN variation across the 90 mm width can exceed 15 FN units, which is unacceptable for applications requiring uniform corrosion resistance. Countermeasures include:

• Using a base metal preheat temperature of 150–250°C to reduce thermal gradient across the width
• Implementing interpass temperature control between 100–200°C for multi-pass deposition
• Employing a high-Ni wire composition (Ni equivalent >14) to compensate for dilution
• Using a backing plate or thermal mass block at the edges to homogenize cooling rates
• Applying a final solution heat treatment (1050–1100°C, water quench) to dissolve carbides and homogenize the microstructure

Engineering Practice and Inspection Considerations

In pressure vessel fabrication, the acceptance criteria for clad plate weld overlay are governed by standards such as GB/T 150, NB/T 47002, and ASME VIII Div.1 Appendix M. The bond strength of the overlay to the base metal must be verified by bend tests, and the overlay thickness must be measured by magnetic thickness gauge or ultrasonic testing (UT) in accordance with JB/T 4730.

For the 90 mm wide belt process, additional inspection requirements include:

• FN measurement at multiple positions across the belt width (minimum 5 points: center, 20 mm each side, 40 mm each side)
• Chemical analysis of the overlay at the same positions to verify dilution levels
• Hardness testing across the width to detect microstructural variation
• Intergranular corrosion testing (ASTM A923 Method 1B or ASTM G150) on representative coupons

The literature highlights a practical case where a 90 mm wide belt overlay of 316L stainless steel on a Q345R base plate exhibited FN values ranging from 8 FN at the center to 38 FN at the edges. The edge regions failed intergranular corrosion testing due to excessive carbide precipitation in the ferrite-rich microstructure. The corrective action involved adjusting the wire composition to a higher Ni grade (Ni equivalent increased from 12 to 16) and implementing edge thermal management using copper backing blocks.

Study Insights and Reflections

The study of 90 mm wide belt extreme cladding reveals that the fundamental challenge is not the deposition of a corrosion-resistant alloy per se, but the uniformity of that deposition across a wide thermal gradient. Conventional narrow-strip cladding benefits from well-established process windows and predictable dilution behavior, but the 90 mm width pushes the process into a regime where thermal management becomes the dominant variable.

From a materials science perspective, the ferrite number is not merely a quality metric but a proxy for the entire microstructural state of the overlay. A high FN indicates not only more delta ferrite but also different carbide precipitation behavior, different phase transformation kinetics, and ultimately different corrosion performance. The connection between FN, dilution, and corrosion resistance is well established in the literature, but the spatial variation of all three parameters across a 90 mm width remains a significant engineering challenge.

The practical implication is that process qualification for wide belt cladding must include spatially resolved testing, not just point sampling. A single FN measurement at the center of the belt provides no information about the edge conditions, which are precisely where corrosion failures are most likely to initiate in service. This insight should be incorporated into any future qualification procedure for wide belt cladding processes, and the standards bodies should consider mandating spatially resolved FN and chemical analysis for overlays exceeding 50 mm in width.

The broader lesson is that as cladding processes scale up in width to improve productivity, the thermal and metallurgical complexity increases non-linearly, and the quality assurance framework must evolve accordingly. The 90 mm wide belt extreme cladding process represents a promising productivity improvement, but its successful implementation requires a deep understanding of the spatial variation in microstructure and properties, which is the central contribution of the literature under review.